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<h3><a id="protocol" href="#protocol">Kafka Wire Protocol</a></h3>

<p>This document covers the wire protocol implemented in Kafka. It is meant to give a readable guide to the protocol
    that covers the available requests, their binary format, and the proper way to make use of them to implement a
    client. This document assumes you understand the basic design and terminology described <a
            href="https://kafka.apache.org/documentation.html#design">here</a></p>

<ul class="toc">
    <li><a href="#protocol_preliminaries">Preliminaries</a>
        <ul>
            <li><a href="#protocol_network">Network</a>
            <li><a href="#protocol_partitioning">Partitioning and bootstrapping</a>
            <li><a href="#protocol_partitioning_strategies">Partitioning Strategies</a>
            <li><a href="#protocol_batching">Batching</a>
            <li><a href="#protocol_compatibility">Versioning and Compatibility</a>
        </ul>
    </li>
    <li><a href="#protocol_details">The Protocol</a>
        <ul>
            <li><a href="#protocol_types">Protocol Primitive Types</a>
            <li><a href="#protocol_grammar">Notes on reading the request format grammars</a>
            <li><a href="#protocol_common">Common Request and Response Structure</a>
            <li><a href="#protocol_message_sets">Message Sets</a>
        </ul>
    </li>
    <li><a href="#protocol_constants">Constants</a>
        <ul>
            <li><a href="#protocol_error_codes">Error Codes</a>
            <li><a href="#protocol_api_keys">Api Keys</a>
        </ul>
    </li>
    <li><a href="#protocol_messages">The Messages</a></li>
    <li><a href="#protocol_philosophy">Some Common Philosophical Questions</a></li>
</ul>

<h4><a id="protocol_preliminaries" href="#protocol_preliminaries">Preliminaries</a></h4>

<h5><a id="protocol_network" href="#protocol_network">Network</a></h5>

<p>Kafka uses a binary protocol over TCP. The protocol defines all apis as request response message pairs. All messages
    are size delimited and are made up of the following primitive types.</p>

<p>The client initiates a socket connection and then writes a sequence of request messages and reads back the
    corresponding response message. No handshake is required on connection or disconnection. TCP is happier if you
    maintain persistent connections used for many requests to amortize the cost of the TCP handshake, but beyond this
    penalty connecting is pretty cheap.</p>

<p>The client will likely need to maintain a connection to multiple brokers, as data is partitioned and the clients will
    need to talk to the server that has their data. However it should not generally be necessary to maintain multiple
    connections to a single broker from a single client instance (i.e. connection pooling).</p>

<p>The server guarantees that on a single TCP connection, requests will be processed in the order they are sent and
    responses will return in that order as well. The broker's request processing allows only a single in-flight request
    per connection in order to guarantee this ordering. Note that clients can (and ideally should) use non-blocking IO
    to implement request pipelining and achieve higher throughput. i.e., clients can send requests even while awaiting
    responses for preceding requests since the outstanding requests will be buffered in the underlying OS socket buffer.
    All requests are initiated by the client, and result in a corresponding response message from the server except
    where noted.</p>

<p>The server has a configurable maximum limit on request size and any request that exceeds this limit will result in
    the socket being disconnected.</p>

<h5><a id="protocol_partitioning" href="#protocol_partitioning">Partitioning and bootstrapping</a></h5>

<p>Kafka is a partitioned system so not all servers have the complete data set. Instead recall that topics are split
    into a pre-defined number of partitions, P, and each partition is replicated with some replication factor, N. Topic
    partitions themselves are just ordered "commit logs" numbered 0, 1, ..., P.</p>

<p>All systems of this nature have the question of how a particular piece of data is assigned to a particular partition.
    Kafka clients directly control this assignment, the brokers themselves enforce no particular semantics of which
    messages should be published to a particular partition. Rather, to publish messages the client directly addresses
    messages to a particular partition, and when fetching messages, fetches from a particular partition. If two clients
    want to use the same partitioning scheme they must use the same method to compute the mapping of key to
    partition.</p>

<p>These requests to publish or fetch data must be sent to the broker that is currently acting as the leader for a given
    partition. This condition is enforced by the broker, so a request for a particular partition to the wrong broker
    will result in an the NotLeaderForPartition error code (described below).</p>

<p>How can the client find out which topics exist, what partitions they have, and which brokers currently host those
    partitions so that it can direct its requests to the right hosts? This information is dynamic, so you can't just
    configure each client with some static mapping file. Instead all Kafka brokers can answer a metadata request that
    describes the current state of the cluster: what topics there are, which partitions those topics have, which broker
    is the leader for those partitions, and the host and port information for these brokers.</p>

<p>In other words, the client needs to somehow find one broker and that broker will tell the client about all the other
    brokers that exist and what partitions they host. This first broker may itself go down so the best practice for a
    client implementation is to take a list of two or three urls to bootstrap from. The user can then choose to use a
    load balancer or just statically configure two or three of their kafka hosts in the clients.</p>

<p>The client does not need to keep polling to see if the cluster has changed; it can fetch metadata once when it is
    instantiated cache that metadata until it receives an error indicating that the metadata is out of date. This error
    can come in two forms: (1) a socket error indicating the client cannot communicate with a particular broker, (2) an
    error code in the response to a request indicating that this broker no longer hosts the partition for which data was
    requested.</p>
<ol>
    <li>Cycle through a list of "bootstrap" kafka urls until we find one we can connect to. Fetch cluster metadata.</li>
    <li>Process fetch or produce requests, directing them to the appropriate broker based on the topic/partitions they
        send to or fetch from.
    </li>
    <li>If we get an appropriate error, refresh the metadata and try again.</li>
</ol>

<h5><a id="protocol_partitioning_strategies" href="#protocol_partitioning_strategies">Partitioning Strategies</a></h5>

<p>As mentioned above the assignment of messages to partitions is something the producing client controls. That said,
    how should this functionality be exposed to the end-user?</p>

<p>Partitioning really serves two purposes in Kafka:</p>
<ol>
    <li>It balances data and request load over brokers</li>
    <li>It serves as a way to divvy up processing among consumer processes while allowing local state and preserving
        order within the partition. We call this semantic partitioning.
    </li>
</ol>

<p>For a given use case you may care about only one of these or both.</p>

<p>To accomplish simple load balancing a simple approach would be for the client to just round robin requests over all
    brokers. Another alternative, in an environment where there are many more producers than brokers, would be to have
    each client chose a single partition at random and publish to that. This later strategy will result in far fewer TCP
    connections.</p>

<p>Semantic partitioning means using some key in the message to assign messages to partitions. For example if you were
    processing a click message stream you might want to partition the stream by the user id so that all data for a
    particular user would go to a single consumer. To accomplish this the client can take a key associated with the
    message and use some hash of this key to choose the partition to which to deliver the message.</p>

<h5><a id="protocol_batching" href="#protocol_batching">Batching</a></h5>

<p>Our apis encourage batching small things together for efficiency. We have found this is a very significant
    performance win. Both our API to send messages and our API to fetch messages always work with a sequence of messages
    not a single message to encourage this. A clever client can make use of this and support an "asynchronous" mode in
    which it batches together messages sent individually and sends them in larger clumps. We go even further with this
    and allow the batching across multiple topics and partitions, so a produce request may contain data to append to
    many partitions and a fetch request may pull data from many partitions all at once.</p>

<p>The client implementer can choose to ignore this and send everything one at a time if they like.</p>

<h5><a id="protocol_compatibility" href="#protocol_compatibility">Versioning and Compatibility</a></h5>

<p>The protocol is designed to enable incremental evolution in a backward compatible fashion. Our versioning is on a
    per-api basis, each version consisting of a request and response pair. Each request contains an API key that
    identifies the API being invoked and a version number that indicates the format of the request and the expected
    format of the response.</p>

<p>The intention is that clients would implement a particular version of the protocol, and indicate this version in
    their requests. Our goal is primarily to allow API evolution in an environment where downtime is not allowed and
    clients and servers cannot all be changed at once.</p>

<p>The server will reject requests with a version it does not support, and will always respond to the client with
    exactly the protocol format it expects based on the version it included in its request. The intended upgrade path is
    that new features would first be rolled out on the server (with the older clients not making use of them) and then
    as newer clients are deployed these new features would gradually be taken advantage of.</p>

<p>Currently all versions are baselined at 0, as we evolve these APIs we will indicate the format for each version
    individually.</p>

<h5><a id="api_versions" href="#api_versions">Retrieving Supported API versions</a></h5>
<p>In order for a client to successfully talk to a broker, it must use request versions supported by the broker. Clients
    may work against multiple broker versions, however to do so the clients need to know what versions of various APIs a
    broker supports. Starting from 0.10.0.0, brokers provide information on various versions of APIs they support.
    Details
    of this new capability can be found <a
            href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-35+-+Retrieving+protocol+version">here</a>.
    Clients may use the supported API versions information to take appropriate actions such as propagating an
    unsupported
    API version error to application or choose an API request/response version supported by both the client and broker.
    The following sequence maybe used by a client to obtain supported API versions from a broker.</p>
<ol>
    <li>Client sends <code>ApiVersionsRequest</code> to a broker after connection has been established with the broker.
        If SSL is enabled,
        this happens after SSL connection has been established.
    </li>
    <li>On receiving <code>ApiVersionsRequest</code>, a broker returns its full list of supported ApiKeys and
        versions regardless of current authentication state (e.g., before SASL authentication on an SASL listener, do
        note that no
        Kafka protocol requests may take place on a SSL listener before the SSL handshake is finished). If this is
        considered to
        leak information about the broker version a workaround is to use SSL with client authentication which is
        performed at an
        earlier stage of the connection where the <code>ApiVersionRequest</code> is not available. Also, note that
        broker versions older
        than 0.10.0.0 do not support this API and will either ignore the request or close connection in response to the
        request.
    </li>
    <li>If multiple versions of an API are supported by broker and client, clients are recommended to use the latest
        version supported
        by the broker and itself.
    </li>
    <li>Deprecation of a protocol version is done by marking an API version as deprecated in protocol documentation.
    </li>
    <li>Supported API versions obtained from a broker, is valid only for current connection on which that information is
        obtained.
        In the event of disconnection, the client should obtain the information from broker again, as the broker might
        have
        upgraded/downgraded in the mean time.
    </li>
</ol>


<h5><a id="sasl_handshake" href="#sasl_handshake">SASL Authentication Sequence</a></h5>
<p>The following sequence is used for SASL authentication:
<ol>
    <li>Kafka <code>ApiVersionsRequest</code> may be sent by the client to obtain the version ranges of requests
        supported by the broker. This is optional.
    </li>
    <li>Kafka <code>SaslHandshakeRequest</code> containing the SASL mechanism for authentication is sent by the client.
        If the requested mechanism is not enabled
        in the server, the server responds with the list of supported mechanisms and closes the client connection. If
        the mechanism is enabled
        in the server, the server sends a successful response and continues with SASL authentication.
    <li>The actual SASL authentication is now performed. A series of SASL client and server tokens corresponding to the
        mechanism are sent as opaque
        packets. These packets contain a 32-bit size followed by the token as defined by the protocol for the SASL
        mechanism.
    <li>If authentication succeeds, subsequent packets are handled as Kafka API requests. Otherwise, the client
        connection is closed.
</ol>
<p>For interoperability with 0.9.0.x clients, the first packet received by the server is handled as a SASL/GSSAPI client
    token if it is not a valid
    Kafka request. SASL/GSSAPI authentication is performed starting with this packet, skipping the first two steps
    above.</p>


<h4><a id="protocol_details" href="#protocol_details">The Protocol</a></h4>

<h5><a id="protocol_types" href="#protocol_types">Protocol Primitive Types</a></h5>

<p>The protocol is built out of the following primitive types.</p>

<p><b>Fixed Width Primitives</b><p>

<p>int8, int16, int32, int64 - Signed integers with the given precision (in bits) stored in big endian order.</p>

<p><b>Variable Length Primitives</b><p>

<p>bytes, string - These types consist of a signed integer giving a length N followed by N bytes of content. A length of
    -1 indicates null. string uses an int16 for its size, and bytes uses an int32.</p>

<p><b>Arrays</b><p>

<p>This is a notation for handling repeated structures. These will always be encoded as an int32 size containing the
    length N followed by N repetitions of the structure which can itself be made up of other primitive types. In the BNF
    grammars below we will show an array of a structure foo as [foo].</p>

<h5><a id="protocol_grammar" href="#protocol_grammar">Notes on reading the request format grammars</a></h5>

<p>The <a href="https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_Form">BNF</a>s below give an exact context free
    grammar for the request and response binary format. The BNF is intentionally not compact in order to give
    human-readable name. As always in a BNF a sequence of productions indicates concatenation. When there are multiple
    possible productions these are separated with '|' and may be enclosed in parenthesis for grouping. The top-level
    definition is always given first and subsequent sub-parts are indented.</p>

<h5><a id="protocol_common" href="#protocol_common">Common Request and Response Structure</a></h5>

<p>All requests and responses originate from the following grammar which will be incrementally describe through the rest
    of this document:</p>

<pre>
RequestOrResponse => Size (RequestMessage | ResponseMessage)
Size => int32
</pre>

<table class="data-table">
    <tbody>
    <tr>
        <th>Field</th>
        <th>Description</th>
    </tr>
    <tr>
        <td>message_size</td>
        <td>The message_size field gives the size of the subsequent request or response message in bytes. The client can
            read requests by first reading this 4 byte size as an integer N, and then reading and parsing the subsequent
            N bytes of the request.
        </td>
    </tr>
</table>

<h5><a id="protocol_message_sets" href="#protocol_message_sets">Message Sets</a></h5>

<p>A description of the message set format can be found <a
        href="https://cwiki.apache.org/confluence/display/KAFKA/A+Guide+To+The+Kafka+Protocol#AGuideToTheKafkaProtocol-Messagesets">here</a>.
    (KAFKA-3368)</p>

<h4><a id="protocol_constants" href="#protocol_constants">Constants</a></h4>

<h5><a id="protocol_error_codes" href="#protocol_error_codes">Error Codes</a></h5>
<p>We use numeric codes to indicate what problem occurred on the server. These can be translated by the client into
    exceptions or whatever the appropriate error handling mechanism in the client language. Here is a table of the error
    codes currently in use:</p>
<!--#include virtual="generated/protocol_errors.html" -->

<h5><a id="protocol_api_keys" href="#protocol_api_keys">Api Keys</a></h5>
<p>The following are the numeric codes that the ApiKey in the request can take for each of the below request types.</p>
<!--#include virtual="generated/protocol_api_keys.html" -->

<h4><a id="protocol_messages" href="#protocol_messages">The Messages</a></h4>

<p>This section gives details on each of the individual API Messages, their usage, their binary format, and the meaning
    of their fields.</p>
<!--#include virtual="generated/protocol_messages.html" -->

<h4><a id="protocol_philosophy" href="#protocol_philosophy">Some Common Philosophical Questions</a></h4>

<p>Some people have asked why we don't use HTTP. There are a number of reasons, the best is that client implementors can
    make use of some of the more advanced TCP features--the ability to multiplex requests, the ability to simultaneously
    poll many connections, etc. We have also found HTTP libraries in many languages to be surprisingly shabby.</p>

<p>Others have asked if maybe we shouldn't support many different protocols. Prior experience with this was that it
    makes it very hard to add and test new features if they have to be ported across many protocol implementations. Our
    feeling is that most users don't really see multiple protocols as a feature, they just want a good reliable client
    in the language of their choice.</p>

<p>Another question is why we don't adopt XMPP, STOMP, AMQP or an existing protocol. The answer to this varies by
    protocol, but in general the problem is that the protocol does determine large parts of the implementation and we
    couldn't do what we are doing if we didn't have control over the protocol. Our belief is that it is possible to do
    better than existing messaging systems have in providing a truly distributed messaging system, and to do this we
    need to build something that works differently.</p>

<p>A final question is why we don't use a system like Protocol Buffers or Thrift to define our request messages. These
    packages excel at helping you to managing lots and lots of serialized messages. However we have only a few messages.
    Support across languages is somewhat spotty (depending on the package). Finally the mapping between binary log
    format and wire protocol is something we manage somewhat carefully and this would not be possible with these
    systems. Finally we prefer the style of versioning APIs explicitly and checking this to inferring new values as
    nulls as it allows more nuanced control of compatibility.</p>

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